I am sorry, but i am not quite sure I understand your post. Are you asking for antialiasing support in EMF export? If so, I am afraid that does not make too much sense (to me). EMF is (as you know) a vector graphics format. Therefore antialiasing would have to be a feature of the viewer application (MS Word in your case), not the generator application (yEd). (Try to think of an EMF file as a set of drawing instructions, like "draw a line", "draw a curve". How these instructions are realized is - more or less - up to the viewer application.)
I supouse that it was an option in exported EMF file. If you search "emf antialiasing" in google there is a lot of forums with questions similar to this, and there is a lot of confusion with antialiased EMF. Of course there are many answers that say "is an option in the renderer", i don't know, but only EMF files from yEd aren't displayed antialiased into my MS WORD.